Sunday, January 10, 2010

One cannot demonstrate by any experience whatever that the heavens move with diurnal motion; whatever the fact may be, assuming that the heavens move and the earth does not or that the earth moves and the heavens do not, to an eye in the heavens which could see the earth clearly it would appear to move; if the eye were on the earth, the heavens would appear to move’.

Nicole Oresme

It is one thing to propose that the earth rotates upon it’s axis and orbits around the sun. We know that Aristarchus of Samos and Seleucus of Babylon attempted to do this in antiquity without very much success. It is quite another to answer the arguments against such a proposal, many of them formidable. Accordingly there is no evidence of anyone in the Middle Ages trying to say that the earth moved around the sun with an annual motion. This breakthrough would not come until Copernicus’s De revolutionibus and even then, few contemporary astronomers espoused his proposals.

Nethertheless, we do find people in the Middle Ages talking about diurnal motion of the earth, a very contentious issue because of the absence of any visible effects of the earth’s rotation on solid objects; or even the clouds above the earth’s surface. The first indication of this is a curious comment by William of Conches, the Chartrean master in the first half of the 12th century who was very interested in natural philosophy. In his Dragmaticon Philosophie between William as the philosopher and the Duke of Normandy, the Duke asks him what he thinks about this idea that the earth is in motion. William replies saying that:

‘you must be under the influence of that crazed philosopher who is always in a frenzy before lunch and drunk after lunch’

..the implication being that because this unnamed scholar is drunk all the time he feels the earth moving under his feet. The Duke presses William of Conches for a better reason for proposing the stability of the earth, so the philosopher says that the earth is made of earth, and earth is not a mobile element like fire. Therefore it makes no sense to have the earth in motion. Taking a common sense view, who would actually think that the earth is actually moving since we don’t feel it move at all. The mystery here is about whom is William talking about. All we can say is that it looks like someone was raising this question in the twelfth century (and was written off as a drunkard)

The topic comes up again in the fourteenth century under the guidance of Jean Buridan and Nicole Oresme. Buridan noted that, as observers, we can only ever observe relative motions. We cannot really know absolute motions. So if, for example, we happened to be in a boat going along a coastline, we really don’t know whether the boat we are in is moving or if the coastline is moving alongside us. Nonetheless, Buridan rejects the idea that the earth is rotating, based on the fact that if you shoot an arrow straight up in the air, it falls back to the ground exactly where you shoot it from. Clearly if the earth were actually rotating, the arrow should be left behind as the earth rotates under it.

His younger contemporary Nicole Oresme later argued that the arrow experiment doesn’t prove anything. This is because the arrow also has an impressed horizontal force which it picks up as it is shot from the earth which keeps it moving along with the earth. Moreover, Oresme uses a second argument based on what things look like on a moving ship (this is similar to the arguments that Galileo would use 250 years later). Oresme says that if we are on a ship which is moving and we move our hand up the mast and down the mast, for any observer which happens to be on the ship, the hand looks like it is moving straight up and straight down. However, to an observer who is not on the ship, it looks like there is also a horizontal motion. They would see the combination of the ship sailing along and the motion of the hand. The observer would therefore see two different things from two different vantage points. Moreover all the motions that we can do on the moving ship look the same to the sailors on the ship, whether the ship is moored in a dock or is saling fast on the ocean. What Oresme is doing here is something we would recognise today as ‘frames of reference’. This is important in physics of motion. Oresme also notes that it is much more economical if the earth turns once every 24 hours, rather than having every other celestial sphere do that motion themselves.

In the end however, after a long logical disputation, Oresme rejects the possibility of a moving earth. Before doing this he concludes that there is simply no way to decide the issue via logic and reason and therefore falls back on the more common sense, Aristotelian and Biblically supported answer. In a sense Oresme’s decision here is about the wider issue of using reason to answer very difficult questions where the evidence is ambiguous In one passage he says:

‘What I have said here, by way of diversion of intellectual exercise can in this manner serve as a valuable means of refuting and checking those who would like to impugn our faith by argument.’

By this he means that reason is a very powerful thing, but there are limits to what reason can actually show us, like it cannot tell us whether the earth moves or not. Similarly, if it cannot answer a physical question about the world, we have to be very careful about the use of reason in discussions about theological articles of faith. Oresme has therefore used rational arguments to show the potential insufficiency of rational argument.

Although Buridan and Oresme had concluded that the earth did not have diurnal motion, their arguments did reduce the debate to a stalemate and had removed most of the objections. Arguments strikingly similar to theirs would later appear in the work of Copernicus in defence of the heliocentric system, a fact which seems to be lost on 'know it all' Amazon reviewer Viktor Blasjo, who says:

Contemporary fashion requires that Buridan and Oresme be called "brilliant" and "even more brilliant" respectively (p. 66), when they were in fact perpetuating anti-science by maintaining that the question of the earth's rotation "is scientifically indeterminate" (p. 68), and should be decided on the basis of the bible, the "Aristotelian principle that rest is a nobler state than motion" (p. 65), or whatnot. As above, then, they turn away from science towards arbitrary speculation (though admittedly while claiming that they do the opposite).

Now what was that I was saying about it being a bad idea to project your own personal ideology back onto the past?

3 comments:

Thanks for this excellent post. I'd like to make two small points, though.

(1) You write that the helicentric view "would not come until Copernicus’s De revolutionibus". That's not quite true. At least three decades before his masterwork, Copernicus wrote a short treatise, later known as Commentariolus, in which he espoused a heliocentric theory. The treatise was circulated in a wider circle of his friends and made a name for Copernicus. The date of the treatise is a subject of controversy, but it was certainly written before 1514.

(2) You write that "Buridan rejects the idea that the earth is rotating, based on the fact that if you shoot an arrow straight up in the air, it falls back to the ground exactly where you shoot it from." Obviously, in this Buridan is closely following his Aristotle: De Caelo II.14 296b23-25.

'It is clear, then, that the earth must be at the centre and immovable, not only for the reasons already given, but also because heavy bodies forcibly thrown quite straight upward return to the point from which they started, even if they are thrown to an infinite distance. From these considerations then it is clear that the earth does not move and does not lie elsewhere than at the centre.'

..which is a form of the argument Buridan was eventually persuaded by.

Welcome to Quodlibeta

The Latin word Quodlibeta means "whatever you like" and refers to the special occasions at medieval universities when the students (or clerks as they were known) could test their masters by asking any question they fancied. This blog is primarily concerned with religion, science, history and their interface. But like the medieval clerks, we reserve the right to post on anything we want.

I am a historian of a very middling and amateurish sort. Having taken a Masters in Modern History at the University of St Andrews (2004), I completed a law conversion course at Nottingham Law School and embarked upon what has been an exciting career in legal technology. My chief interests are in Twentieth Century History, Colonial America, Historiography and, most recently, the History of Science. I also write a satirical blog at The Diary of Humphrey Clarke. I hope my contributions to Quodlibeta capture something of the pleasure I get from trawling my dusty collection of second hand history books.

Jim S

I have a Ph.D. in Philosophy and Master's degrees in Philosophy and Theology. I'm a generalist, my areas of expertise being epistemology, logic, philosophy of mind, philosophy of religion, and science and religion. I did my philosophical studies in Continental Europe, and currently reside in the Pacific Northwest. I also write the Agent Intellect blog. In my mid-20s I decided to refute Christianity to set my conscience at ease. Two subjects I turned to in particular were science and philosophy. I accidentally argued myself into it.